the Electrical Conduction produced by it. 127 



proportional to the intensity of the radiation (as is the case 

 in Rontgen-ray conduction), and that the intensity of the 

 radiation near the uranium surface is constant over a plane 

 parallel to that surface. This is very approximately the case 

 if the distance from the uranium surface is small compared 

 with the diameter of the radiating surface. 



For simplicity we will consider the case of an infinite 

 plane of uranium giving out homogeneous radiation. 



If I be the intensity of the radiation close to the uranium 

 surface, the intensity at a distance x is equal to le~ kx where \ 

 is the coefficient of absorption of the gas. The intensity is 

 diminished in passing through the layer of aluminium foil A 

 (fig. 3) in a constant ratio for all distances from the uranium. 

 The intensity at a distance x after passing through the 

 aluminium is thus Kle~ Kx where a: is a constant. The rate of 

 production of the ions between two parallel planes between 

 A and B (fig. 3) at distances x + dx and x from the uranium 

 is therefore proportional to /cle~ xx dx. If r be the distance of 

 A from the uranium, and / the distance between A and B, 

 the total number of ions produced per second between A and 

 B is proportional to 



l+r 



I/ce~ kx dx, 



I 



or to 



^T^{l-e-**}. 



When a " saturating " electromotive force (see § 16) acts 

 between A and B, the current is proportional to the total 

 number of ions produced. Now, as the system AB is moved 



fcT. 



from the radiating surface, — (1 — e~ xl ) is a constant for any 



particular gas. We thus see that the rate of leak is propor- 

 tional to e~ kr , or the rate of leak decreases in geometrical 

 progression as the distance r increases in arithmetical pro- 

 gression. 



This result allows us to at once deduce the value of the 

 coefficient of absorption for different gases from the data we 

 have previously given. 



The results are given in the following table : — 



